Magnetic resonance imaging system capable of rapid field ramping

ABSTRACT

Systems and methods for rapidly ramping the magnetic field of a superconducting magnet, such as a superconducting magnet adapted for use in a magnetic resonance imaging system, are provided. The magnetic field can be rapidly ramped up or down by changing the current density in the superconducting magnet while monitoring and controlling the superconducting magnet&#39;s temperature to remain below a transition temperature. A superconducting switch is used to connect the superconducting magnet and a power supply in a connected circuit. The current generated by the power supply is then adjusted to increase or decrease the current density in the superconducting magnet to respectively ramp up or ramp down the magnetic field strength in a controlled manner. The ramp rate at which the magnetic field strength is changed is determined and optimized based on the operating parameters of the superconducting magnet and the current being generated by the power supply.

BACKGROUND OF THE INVENTION

The field of the invention is systems and methods for magnetic resonanceimaging (“MRI”). More particularly, the invention relates to systems andmethods for MRI in which the magnetic field of the MRI scanner can berapidly ramped up and down as needed.

MRI systems typically utilize one of two types of magnet assemblies togenerate the strong, main magnetic field used for imaging. One typegenerates the main magnetic field using permanent magnets. This type ofsystem is less popular because the magnetic field strengths that can beachieved with such systems is limited. Moreover, these systems tend tobe extremely heavy and are very sensitive to temperature fluctuations.Permanent magnets also cannot be turned off, so there is no way toremove the magnetic field.

The second type of MRI system generates the main magnetic field using asuperconducting electromagnet. Using superconducting magnets allows highcurrent densities through the conductors of the electromagnet withoutpower dissipation, which in turn enables the ability to achieve highmagnetic field strengths. For the magnet to be superconducting, themagnet coils must be cooled to extremely low temperatures (e.g., about 4K).

One method used to cool the superconducting magnet coils to this lowtemperature is done by immersing the conductor in a liquid helium bath.These superconducting systems tend to be very expensive because of thehigh cost of the liquid cryogens (e.g., liquid helium). Furthermore, itis not easy to rapidly turn on or off the magnetic fields generated bythese systems. For example, to rapidly turn off the magnetic fieldtypically requires heating up the conductive magnet coils so that theydevelop resistance that can dissipate their stored energy. Thisresistance produces heat that causes the liquid cryogen, which isproviding the cooling, to convert to rapidly expanding gas. Thisboiling-off of the liquid cryogen removes the cooling capability of thesystem, and thus the magnetic field generated by the magnet coils. But,the magnetic field cannot be regenerated until the liquid cryogen isreplaced and the magnet coils are cooled back down to superconductingtemperatures, a process that normally involves multiple days andsignificant expense.

Alternatively, current can be removed or added to superconducting magnetsystems very slowly without causing enough heating to boil off theliquid cryogen. In these situations, it takes many hours to completelyadd or remove the current, making rapid turning the magnetic field on oroff in this manner not feasible.

For safety reasons, it would be beneficial for an MRI scanner to becapable of having the magnetic field rapidly turned off. For example,large metallic objects being attracted by the strong magnetic field isone of the primary risks associated with these devices. Traditionalsuperconducting magnets have implemented a mechanism to rapidly turn offthe magnetic field in an emergency situation by “quenching” the magnetin the manner described above, where all liquid cryogens are boiled offvery rapidly. Quenching the magnet, however, requires a time consumingand expensive replacement of the liquid cryogens and before the magneticfield can be reestablished.

The ability to rapidly ramp up and down the magnetic field of an MRIsystem without the significant expense of losing and replacing expensiveliquid cryogens would be very useful for interventional and mobileimaging applications. In these situations, it would be advantageous toramp down the magnetic field of the MRI system so it could be safelystored (e.g., in a surgical suite) or transported, while at the sametime allowing for the magnetic field to be rapidly ramped up (e.g.,within a matter of minutes) for use as needed.

SUMMARY OF THE INVENTION

The present invention overcomes the aforementioned drawbacks byproviding systems and methods for rapidly ramping up or down themagnetic field generated by a superconducting magnet, such as asuperconducting magnet adapted for use in a magnetic resonance imaging(“MRI”) system.

It is thus one aspect of the invention to provide a control system forramping a magnetic field of a superconducting magnet that is in thermalcontact with a mechanical cryocooler. The control system generallyincludes a superconducting switch that selectively connects asuperconducting magnet to a power supply, and has an open state and aclosed state. When in the closed state, the superconducting switchconnects the superconducting magnet and the power supply in a connectedcircuit. The control system also includes a controller programmed toramp a magnetic field generated by the superconducting magnet from apresent magnetic field strength to a target magnetic field strength byselecting a ramp function defining at least one ramp rate; setting acurrent generated by the power supply to an initial current value;activating the superconducting switch to its closed position, therebyconnecting the superconducting magnet and the power supply in theconnected circuit; adjusting the current generated by the power supplyaccording to the selected ramp function; and activating thesuperconducting switch to its open position when the target magneticfield strength is reached, thereby disconnecting the superconductingmagnet and the power supply from the connected circuit and placing thesuperconducting magnet in a closed circuit.

It is another aspect of the invention to provide a method forcontrolling a ramp-down or a ramp-up of a superconducting magnet. Themethod includes monitoring at least one operating parameter valueindicative of a present state of a superconducting magnet that generatesa magnetic field having a present magnetic field strength anddetermining a ramp function based on the at least one operatingparameter value. Instructions are then provided to a controller to rampthe magnetic field generated by the superconducting magnet based on theat least one operating parameter value by selectively activating asuperconducting switch to connect the superconducting magnet to a powersupply in a connected circuit. When in the connected circuit, a currentgenerated by the power supply is adjusted according to the determinedramp function to adjust the magnetic field generated by thesuperconducting magnet from the present magnetic field strength to atarget magnetic field strength.

It is another aspect of the invention to provide an MRI system whosemagnetic field can be rapidly ramped down and up. The MRI systemgenerally includes magnet coils, a power supply, a superconductingswitch, a mechanical cryocooler, and a controller. The magnet coilsgenerate a magnetic field and are composed of a superconductingmaterial. The superconducting switch selectively connects the magnetcoils to the power supply, and has an open state and a closed state.When in the closed state, the superconducting switch connects the magnetcoils and the power supply in a connected circuit. The mechanicalcryocooler is in thermal contact with the magnet coils and is operableto reduce and maintain a temperature of the magnet coils below atransition temperature of the superconducting material. The controllerprogrammed to ramp the magnetic field generated by the magnet coils froma present magnetic field strength to a target magnetic field strength byselecting a ramp function defining at least one ramp rate; setting acurrent generated by the power supply to an initial current value;activating the superconducting switch to its closed position, therebyconnecting the magnet coils and the power supply in the connectedcircuit; adjusting the current generated by the power supply accordingto the selected ramp function; and activating the superconducting switchto its open position when the target magnetic field strength is reached,thereby disconnecting the magnet coils and the power supply from theconnected circuit and placing the magnet coils in a closed circuit.

The foregoing and other aspects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings that form a part hereof, and in whichthere is shown by way of illustration a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference is made therefore to the claimsand herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example magnetic resonance imaging(“MRI”) system capable of rapid magnetic field ramping.

FIG. 2 is a flowchart setting forth the steps of an example method forcontrolling an MRI system, such as the MRI system of FIG. 1, to rapidlyramp the magnetic field strength of the MRI system.

FIG. 3 is an example ramp function that includes a first ramp periodduring which the temperature of a superconducting magnet is increased toa temperature below a threshold temperature, and a second ramp periodduring which the temperature of the superconducting magnet is decreased,and wherein the one or more ramp rates defining the second ramp periodare slower than the one or more ramp rates defining the first rampperiod.

FIG. 4 is another example ramp function that includes a first rampperiod during which the temperature of a superconducting magnet isincreased to a temperature below a threshold temperature, and a secondramp period during which the temperature of the superconducting magnetis decreased according to a smoothly varying ramp rate that is adjustedbased on a derivative of the temperature.

DETAILED DESCRIPTION OF THE INVENTION

Described here are systems and methods for rapid magnetic field rampingin a magnetic resonance imaging (“MRI”) system that includes asuperconducting magnet cooled by a mechanical cryocooler.

Recently, there have been advances in superconductors andsuperconducting magnet design aimed at reducing the amount of expensiveliquid cryogen required to achieve and maintain superconductingproperties. These advances include the development of high temperaturesuperconductors that are conductors that become superconducting attemperatures higher than 4 K. Currently, reasonable high temperaturesuperconductors can operate at 10 K; although, some materials candemonstrate superconducting properties at temperatures as high as 30 K.Furthermore, there have been recent proposals on cryogen-free magnetdesigns that use a cryocooler to cool the magnet coil conductors throughthermal contact.

The systems and methods described here are based on a mechanicalsuperconducting magnet design using traditional, or high temperature,superconductors where the main magnetic field can be turned on and offin a short amount of time. For instance, the magnetic field can beturned on and off in an amount of time comparable to a typical amount oftime it takes to prepare a subject to be imaged in an MRI system (e.g.,about 10-15 minutes).

The MRI system described here uses a mechanical cryocooler that is inthermal contact with the conductors in a superconducting magnet to coolthem to temperatures approaching 4 K. Here, thermal contact can includedirect or indirect contact, through which thermal energy can betransferred or conducted. The superconducting material used for themagnet design preferably maintains superconducting properties up totemperatures approaching 10 K. In the described system, current densitycan be added or removed from the conductive windings of the magnet coilsin a rapid manner by introducing a power supply source into the circuit(e.g., by means of a superconducting switch). Supplying this current tothe magnet coils introduces heat into the system, which can be removedusing the thermal cooling capacity of the mechanical cryocooler.

In this system, the rate of current change (and thus the rate ofmagnetic field change) can be controlled so that the temperature of theconductor does not exceed the superconducting transition point of themagnet coil material. In this manner, there is no rapid resistancechanges in the conductor to cause an uncontrolled loss of magnetic field(i.e., a quench). Furthermore, the control system described hereprovides a simple user interaction for turning the magnetic field on andoff, monitors the temperature of the conductors during and aftermagnetic field ramping, and is capable of adjusting the ramp function orramp rate, the interval between turning the magnetic field on and off,or both, in order to maintain temperatures that are cold enough tomaintain superconducting properties of the magnet coils.

Referring now to FIG. 1, a magnetic resonance imaging system 10generally includes a magnet assembly 12 for providing a magnetic field14 that is substantially uniform within a bore 16 that may hold asubject 18 or other object to be imaged. The magnet assembly 12 supportsa radio frequency (“RF”) coil (not shown) that may provide an RFexcitation to nuclear spins in the object or subject (not shown)positioned within the bore 16. The RF coil communicates with an RFsystem 20 producing the necessary electrical waveforms, as is understoodin the art.

The magnet assembly 12 also supports three axes of gradient coils (notshown) of a type known in the art, and which communicate with acorresponding gradient system 22 providing electrical power to thegradient coils to produce magnetic field gradients, G_(x), G_(y), andG_(z) over time.

A data acquisition system 24 connects to RF reception coils (not shown)that are supported within the magnet assembly 12 or positioned withinbore 16.

The RF system 20, gradient system 22, and data acquisition system 24each communicates with a controller 26 that generates pulse sequencesthat include RF pulses from the RF system 20 and gradient pulses fromgradient system 22. The data acquisition system 24 receives magneticresonance signals from the RF system 20 and provides the magneticresonance signals to a data processing system 28, which operates toprocess the magnetic resonance signals and to reconstruct imagestherefrom. The reconstructed images can be provided to a display 30 fordisplay to a user.

The magnet assembly 12 includes one or more magnet coils 32 housed in avacuum housing 34, which generally provides a cryostat for the magnetcoils 32, and mechanically cooled by a mechanical cryocooler 36, such asa Gifford-McMahon (“GM”) cryocooler or a pulse tube cryocooler. In oneexample configuration, the cryocooler can be a Model RDK-305Gifford-McMahon cryocooler manufactured by Sumitomo Heavy Industries(Japan). In general, the cryocooler 36 is in thermal contact with themagnet coils 32 and is operable to lower the temperature of the magnetcoils 32 and to maintain the magnet coils 32 and a desired operatingtemperature. In some embodiments the cryocooler 36 includes a firststage in thermal contact with the vacuum housing 34 and a second stagein thermal contact with the magnet coils 32. In these embodiments, thefirst stage of the cryocooler 36 maintains the vacuum housing 34 at afirst temperature and the second stage of the cryocooler 36 maintainsthe magnet coils 32 at a second temperature that is lower than the firsttemperature.

The magnet coils 32 are composed of a superconducting material andtherefore provide a superconducting magnet. The superconducting materialis preferably selected to be a material with a suitable criticaltemperature such that the magnet coils 32 are capable of achievingdesired magnetic field strengths over a range of suitable temperatures.As one example, the superconducting material can be niobium (“Nb”),which has a transition temperature of about 9.2 K. As another example,the superconducting material can be niobium-titanium (“NbTi”), which hasa transition temperature of about 10 K. As still another example, thesuperconducting material can be triniobium-tin (“Nb₃Sn”), which has atransition temperature of about 18.3 K.

The choice of superconducting material will define the range of magneticfield strengths achievable with the magnet assembly 12. Preferably, thesuperconducting material is chosen such that magnetic field strengths inthe range of about 0.0 T to about 3.0 T can be achieved over a range oftemperatures that can be suitably achieved by the cryocooler 36. In someconfigurations, however, the superconducting material can be chosen toprovide magnetic field strengths higher than 3.0 T.

The cryocooler 36 is operable to maintain the magnet coils 32 at anoperational temperature at which the magnet coils 32 aresuperconducting, such as a temperature that is below the transition, orcritical, temperature for the material of which the magnet coils 32 arecomposed. As one example, a lower operational temperature limit can beabout 4 K and an upper operational temperature limit can be at or nearthe transition, or critical, temperature of the superconducting materialof which the magnet coils 32 are composed.

The current density in the magnet coils 32 in the MRI system 10 of thepresent invention is controllable to rapidly ramp up or ramp down themagnetic field 14 generated by the magnet assembly 12 while controllingthe temperature of the magnet coils 32 with the cryocooler 36 to keepthe temperature below the transition temperature of the superconductingmaterial of which the magnet coils 32 are composed. As one example, themagnetic field 14 can be ramped up or ramped down on the order ofminutes, such as fifteen minutes or less.

In general, the current density in the magnet coils 32 can be increasedor decreased by connecting the magnet coils 32 to a circuit with a powersupply 38 that is in electrical communication with the magnet coils 32via a switch 40 and operating the power supply 38 to increase ordecrease the current in the connected circuit. The switch 40 isgenerally a superconducting switch that is operable between a first,closed, state and a second, open, state.

When the switch 40 is in its open state, the magnet coils 32 are in aclosed circuit, which is sometimes referred to as a “persistent mode.”In this configuration, the magnet coils 32 are in a superconductingstate so long as the temperature of the magnet coils 32 is maintained ata temperature at or below the transition temperature of thesuperconducting material of which they are composed.

When the switch 40 is in the closed state, however, the magnet coils 32and the power supply 38 can be placed in a connected circuit, and thecurrent supplied by the power supply 38 and the current in the magnetcoils 32 will try to equalize. For instance, if the power supply 38 isoperated to supply more current to the connected circuit, the current inthe magnet coils 32 will increase, which will increase the strength ofthe magnetic field 14. On the other hand, if the power supply 38 isoperated to decrease the current in the connected circuit, the currentin the magnet coils 32 will decrease, which will decrease the strengthof the magnetic field 14.

It will be appreciated by those skilled in the art that any suitablesuperconducting switch can be used for selectively connecting the magnetcoils 32 and power supply 38 into a connected circuit; however, as onenon-limiting example, the switch 40 may include a length ofsuperconducting wire that is connected in parallel to the magnet coils32 and the power supply 38. To operate such a switch 40 into its closedstate, a heater in thermal contact with the switch 40 is operated toraise the temperature of the superconducting wire above its transitiontemperature, which in turn makes the wire highly resistive compared tothe inductive impedance of the magnet coils 32. As a result, very littlecurrent will flow through the switch 40. The power supply 38 can then beplaced into a connected circuit with the magnet coils 32. When in thisconnected circuit, the current in the power supply 38 and the magnetcoils 32 will try to equalize; thus, by adjusting the current suppliedby the power supply 38, the current density in the magnet coils 32 canbe increased or decreased to respectively ramp up or ramp down themagnetic field 14. To operate the switch 40 into its open state, thesuperconducting wire in the switch 40 is cooled below its transitiontemperature, which places the magnet coils 32 back into a closedcircuit, thereby disconnecting the power supply 38 and allowing all ofthe current to flow through the magnet coils 32.

When the magnet coils 32 are in the connected circuit with the powersupply 38, the temperature of the magnet coils 32 will increase as thecurrent in the connected circuit equalizes. Thus, the temperature of themagnet coils 32 should be monitored to ensure that the temperature ofthe magnet coils 32 remains below the transition temperature for thesuperconducting material of which they are composed. Because placing themagnet coils 32 into a connected circuit with the power supply 38 willtend to increase the temperature of the magnet coils 32, the rate atwhich the magnetic field 14 can be ramped up or ramped down will dependin part on the cooling capacity of the cryocooler 36. For instance, acryocooler with a larger cooling capacity will be able to more rapidlyremove heat from the magnet coils 32 while they are in a connectedcircuit with the power supply 38.

The power supply 38 and the switch 40 operate under control from thecontroller 26 to provide current to the magnet coils 32 when the powersupply 38 is in a connected circuit with the magnet coils 32. A currentmonitor 42 measures the current flowing to the magnet coils 32 from thepower supply 38, and a measure of the current can be provided to thecontroller 26 to control the ramping up or ramping down of the magneticfield 14. In some configurations, the current monitor 42 is integratedinto the power supply 38.

A temperature monitor 44 is in thermal contact with the magnet assembly12 and operates to measure a temperature of the magnet coils 32 inreal-time. As one example, the temperature monitor 44 can include athermocouple temperature sensor, a diode temperature sensor (e.g., asilicon diode or a GaAlAs diode), a resistance temperature detector(“RTD”), a capacitive temperature sensor, and so on. RTD-basedtemperature sensors can be composed of ceramic oxynitride, germanium, orruthenium oxide. The temperature of the magnet coils 32 is monitored andcan be provided to the controller 26 to control the ramping up orramping down of the magnetic field 14.

In operation, the controller 26 is programmed to ramp up or ramp downthe magnetic field 14 of the magnet assembly 12 in response toinstructions from a user. As mentioned above, the magnetic field 14 canbe ramped down by decreasing the current density in the magnet coils 32by supplying current to the magnet coils 32 from the power supply 38 viathe switch 40, which is controlled by the controller 26. Likewise, thestrength of the magnetic field 14 can be ramped up by increasing thecurrent density in the magnet coils 32 by supplying current to themagnet coils 32 from the power supply 38 via the switch 40, which iscontrolled by the controller 26.

The controller 26 is also programmed to monitor various operationalparameter values associated with the MRI system 10 before, during, andafter ramping the magnetic field 14 up or down. As one example, asmentioned above, the controller 26 can monitor the current supplied tothe magnet coils 32 by the power supply 38 via data received from thecurrent monitor 42. As another example, as mentioned above, thecontroller 26 can monitor the temperature of the magnet coils 32 viadata received from the temperature monitor 44. As still another example,the controller 26 can monitor the strength of the magnetic field 14,such as by receiving data from a magnetic field sensor, such as a Hallprobe or the like, positioned in or proximate to the bore 16 of themagnet assembly 12.

As will now be described in more detail, the controller 26 canindividually or collectively monitor operational parameter values suchas the current (I) being supplied to the magnet coils 32, thetemperature (T) of the magnet coils 32, and the magnetic field strength(B₀) generated by the magnet coils 32, to control the ramping of themagnetic field 14 up or down.

Referring now to FIG. 2, a flowchart is illustrated as setting forth thesteps of an example method for ramping the magnetic field generated bythe MRI system descried above with respect to FIG. 1. In general, themagnetic field will be ramped up or down based on a set point thatdefines a desired magnetic field strength to which the superconductingmagnet should be ramped.

To this end, a set point is set by the controller 26, as indicated atstep 202. The set point is generally selected based on instructionsprovided by the user, such as a user-defined, target magnetic fieldstrength to which the superconducting magnet should be ramped. Forinstance, the user can provide instructions to the controller 26 to rampthe magnetic field strength down to zero from a present magnetic fieldstrength, thereby “turning off” the magnetic field of the MRI system 10.As another example, the user can provide instruction to the controller26 to ramp down the magnetic field from a first magnetic field strengthto a second magnetic field strength that is weaker than the first. Forinstance, the instructions may be to ramp the magnetic field down from3.0 T to 1.5 T to implement different imaging applications at thedifferent field strengths. Similarly, the instructions can be to ramp upthe magnetic field, such as from 1.5 T to 3.0 T, or from zero field to adesired magnetic field strength.

After the set point has been established, the present operatingparameter values associated with the MRI system 10 are provided to thecontroller 26, as indicated at step 204. Collectively, these operatingparameters describe the present state of the MRI system 10, or ofcomponents within the MRI system 10, such as the magnet assembly 12 orthe magnet coils 32. As one example, the temperature of the magnet coils32 and the present magnetic field strength can be received by thecontroller 26.

A check of the present operating parameter values is made at step 206.For instance, the temperature of the magnet coils 32, the presentmagnetic field strength, or both can be checked to confirm whether theinstructions to ramp the magnetic field should be implemented. As oneexample, if the temperature of the magnet coils 32 is such that it wouldnot be suitable to ramp the magnetic field, then the ramping process isterminated or otherwise placed on hold until the temperature of themagnet coils 32 is appropriately changed. For example, if thetemperature of the magnet coils 32 is at or above the upper operationaltemperature limit (e.g., the critical, or transition, temperature forthe magnet coil superconducting material) of the MRI system 10 then itwould not be suitable to further increase the temperature of the magnetcoils 32 and the magnetic field ramping should be terminated or put onhold until the cryocooler 36 is able to lower the temperature of themagnet coils 32 to a suitable temperature.

A check is then made at step 208 to confirm the present settings of thepower supply 38. For instance, the present settings of the power supply38 are checked to confirm that the current in the power supply 38 isnear a target operating current. When ramping up the magnetic field, thetarget operating current is at or near zero current, and when rampingdown the magnetic field, the target operating current is at or near theexpected current in the magnet coils 32. If the present current in thepower supply 38 is not close to the target operating current, then thesettings of the power supply 38 are adjusted to bring the presentcurrent to the target operating current.

After confirming the present settings of the power supply 38, thecontroller 26 sends instructions to operate the switch 40 to its closedstate such that current can flow from the power supply 38 to the magnetcoils 32, as indicated at step 210. The controller 26 controls theramping of the magnetic field according to a ramp function that definesat least one ramp rate.

With the switch 40 in its closed state, the power supply 38 and magnetcoils 32 are placed into a connected circuit. The current supplied bythe power supply 38 is then slowly increased or decreased torespectively ramp up or ramp down the magnetic field, as indicated atstep 212. While the magnet coils 32 and power supply 38 are in theconnected circuit, the energy change caused by the increase or decreasein the power supply 38 current will generate heat that raises thetemperature of the magnet coils 32. Thus, during the ramping process thetemperature of the magnet coils 32 is monitored, as indicated at step214.

As one example, the present temperature of the magnet coils 32 can bemeasured by the temperature monitor 44 and monitored to confirm whetherthe temperature of the magnet coils 32 is being maintained withinoperational limits (e.g., whether the temperature of the magnet coils 32is being maintained below the transition temperature of thesuperconducting material of which the magnet coils 32 are composed). Inaddition to the temperature, other operating parameter values of the MRIsystem 10 can be monitored, including the magnetic field strength, whichcan be measured by a Hall probe, or other suitable sensor. The magneticfield strength is preferably monitored to confirm whether a magneticfield set point has been reached. As still another example, the currentsupplied by the power supply 38 can be measured by the current monitor42 and monitored to confirm that the appropriate level of current isbeing supplied to the magnet coils 32.

A determination is made at decision block 216 whether the set point hasbeen reached based on the monitoring of the operating parameters of theMRI system 10. If the set point has not been reached, then monitoring ofthe operating parameters continues until the set point has been reached.While the operating parameters are being monitored, a determination isalso made at decision block 218 whether the magnet coils 32 riskoverheating (e.g., being heated near or above the transitiontemperature) based on the rate at which current is being added orremoved from the magnet coils 32. If the temperature of the magnet coils32 is close to being raised above the transition temperature, then thepower supply 38 settings can be adjusted, as indicated at step 220, totemporarily stop changing the current in the connected circuit, or tootherwise slow down the ramp rate. Slowing down, or otherwise stopping,the ramping process will reduce the heating of the magnet coils 32 andallow the cryocooler 36 to lower the temperature of the magnet coils 32to a more suitable temperature. Once the temperature has been reduced toa more suitable level, the settings of the power supply 38 can again beadjusted to continue increasing or decreasing the current density in themagnet coils 32 until the set point is reached.

When the set point is reached, instructions are sent by the controller26 to operate the switch 40 to its open state, as indicated at step 222.After the switch 40 is in its open state, the operating parameters ofthe MRI system 10 are still monitored, as indicated at step 224, untilthe temperature of the magnet coils 32 is reduced to a suitabletemperature for operation, as determined at decision block 226. When themagnet coils 32 reach a stable operating temperature, the MRI system 10is ready to perform a scan, as indicated at step 228.

In some embodiments, the ramp function or ramp rate can be user defined.In some other embodiments, the ramp function or ramp rate can beoptimized to reduce total ramping time based on the present operatingparameters of the MRI system 10 and the set point selected by the user.As shown in FIGS. 3 and 4, the ramp function 50 generally includes afirst ramp period during which the temperature of the magnet coils 32increases to a temperature below a threshold temperature, and a secondramp period during which the temperature of the magnet coils 32decreases. The first ramp period can be defined by an initial ramp rate,and the second ramp period can be defined by one or more ramp rates thatare slower than the initial ramp rate.

Referring particularly now to FIG. 3, the total ramping time can bereduced by using a ramp function 50 that begins with a fast initial ramprate, such that the equilibrium temperature of the magnet coils 32 atthat particular ramp rate does not exceed a predefined thresholdtemperature. Next, once the magnet coils 32 have reached the equilibriumtemperature, or when the current in the magnet coils 32 has reached apredefined level, the ramp rate is decreased by an amount such that themagnet coils 32 will have a new, lower equilibrium temperature. Thisprocess can be repeated until the magnet coils 32 are energized with thedesired current (i.e., reach the target magnetic field strength).

The initial “fast” ramping period, the individual ramp rates, and theramp step size can be adjusted such that one of the following effects isachieved: (1) the desired current, and thus the target magnetic fieldstrength, is reached in the smallest amount of time, or (2) the finalramping equilibrium temperature is close, or substantially similar, tothe equilibrium temperature of the magnet coils 32 in persistent mode.

Furthermore, as illustrated in FIG. 4, the derivative of the temperatureof the magnet coils 32 can be monitored and the ramp function 50 morefrequently adjusted in finer ramp rate step sizes, such that thetemperature decay curve in the ramp function 50 is smoothly varying. Theramp function or ramp rate can also be controlled during the rampingprocess based on a monitoring of the operating parameters, as describedabove.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

What is claimed is:
 1. A control system for ramping a magnetic field of a superconducting magnet that is in thermal contact with a mechanical cryocooler, comprising: a superconducting switch selectively connecting the superconducting magnet to a power supply and having an open state and a closed state, wherein when in the closed state the superconducting switch connects the superconducting magnet and the power supply in a connected circuit; a controller programmed to ramp a magnetic field generated by the superconducting magnet from a present magnetic field strength to a target magnetic field strength by: (i) selecting a ramp function defining at least one ramp rate; (ii) setting a current generated by the power supply to an initial current value; (iii) activating the superconducting switch to its closed position, thereby connecting the superconducting magnet and the power supply in the connected circuit; (iv) adjusting the current generated by the power supply according to the selected ramp function; and (v) activating the superconducting switch to its open position when the target magnetic field strength is reached, thereby disconnecting the superconducting magnet and the power supply from the connected circuit and placing the superconducting magnet in a closed circuit.
 2. The control system as recited in claim 1, wherein the initial current value is zero, and adjusting the current generated by the power supply when the power supply is in the connected circuit comprises increasing the current generated by the power supply.
 3. The control system as recited in claim 1, wherein the initial current value is substantially similar to an expected current in the superconducting magnet, and adjusting the current generated by the power supply when the power supply is in the connected circuit comprises decreasing the current generated by the power supply.
 4. The control system as recited in claim 1, wherein the controller is programmed to receive at least one operating parameter value indicative of a present state of the superconducting magnet and to select the ramp function based on the at least one operating parameter value and the target magnetic field strength.
 5. The control system as recited in claim 4, wherein the at least one operating parameter value is a temperature of the superconducting magnet or the present magnetic field strength of the magnetic field generated by the superconducting magnet.
 6. The control system as recited in claim 5, further comprising a temperature monitor in thermal contact with the superconducting magnet so as to measure the temperature of the superconducting magnet.
 7. The control system as recited in claim 5, wherein the controller is programmed to select the ramp function to at least one of minimize a time required to reach the target magnetic field strength or such that the temperature of the superconducting magnet at the target magnetic field is substantially similar to an equilibrium temperature of the superconducting magnet in a persistent mode.
 8. The control system as recited in claim 7, wherein the ramp function comprises a first ramp period during which the temperature of the superconducting magnet increases according to an initial ramp rate and a second ramp period during which the temperature of the superconducting magnet decreases according to at least one ramp rate that is slower than the initial ramp rate.
 9. A method for controlling a ramp-down or a ramp-up of a superconducting magnet, the steps of the method comprising: (i) monitoring at least one operating parameter value indicative of a present state of the superconducting magnet that generates a magnetic field having a present magnetic field strength; (ii) determining a ramp function based on the at least one operating parameter value; (iii) providing instructions to a controller to ramp the magnetic field generated by the superconducting magnet based on the at least one operating parameter value by selectively activating a superconducting switch to connect the superconducting magnet to a power supply in a connected circuit; and wherein when in the connected circuit, a current generated by the power supply is adjusted according to the determined ramp function to adjust the magnetic field generated by the superconducting magnet from the present magnetic field strength to a target magnetic field strength.
 10. The method as recited in claim 9, wherein the at least one operating parameter value is at least one of a temperature of the superconducting magnet or the present magnetic field strength.
 11. The method as recited in claim 10, wherein the ramp function is determined to at least one of minimize a time required to reach the target magnetic field strength or such that a temperature of the superconducting magnet at the target magnetic field is substantially similar to an equilibrium temperature of the superconducting magnet in a persistent mode.
 12. A magnetic resonance imaging (MRI) system, comprising: magnet coils for generating a magnetic field, wherein the magnet coils are composed of a superconducting material; a power supply; a superconducting switch selectively connecting the magnet coils to the power supply and having an open state and a closed state, wherein when in the closed state the superconducting switch connects the magnet coils and the power supply in a connected circuit; a mechanical cryocooler in thermal contact with the magnet coils and operable to reduce and maintain a temperature of the magnet coils below a transition temperature of the superconducting material; a controller programmed to ramp the magnetic field generated by the magnet coils from a present magnetic field strength to a target magnetic field strength by: (i) selecting a ramp function defining at least one ramp rate; (ii) setting a current generated by the power supply to an initial current value; (iii) activating the superconducting switch to its closed position, thereby connecting the magnet coils and the power supply in the connected circuit; (iv) adjusting the current generated by the power supply according to the selected ramp function; and (v) activating the superconducting switch to its open position when the target magnetic field strength is reached, thereby disconnecting the magnet coils and the power supply from the connected circuit and placing the magnet coils in a closed circuit.
 13. The MRI system as recited in claim 12, wherein the initial current value is zero, and adjusting the current generated by the power supply when the power supply is in the connected circuit comprises increasing the current generated by the power supply.
 14. The MRI system as recited in claim 12, wherein the initial current value is substantially similar to an expected current in the magnet coils, and adjusting the current generated by the power supply when the power supply is in the connected circuit comprises decreasing the current generated by the power supply.
 15. The MRI system as recited in claim 12, wherein the controller is programmed to receive at least one operating parameter value indicative of a present state of the MRI system and to select the ramp function based on the at least one operating parameter value and the target magnetic field strength.
 16. The MRI system as recited in claim 15, further comprising a temperature monitor in thermal contact with the magnet coils so as to measure the temperature of the magnet coils, and wherein the at least one operating parameter value includes the temperature of the magnet coils.
 17. The MRI system as recited in claim 16, wherein the controller is programmed to select the ramp function to at least one of minimize a time required to reach the target magnetic field strength or such that the temperature of the magnet coils at the target magnetic field is substantially similar to an equilibrium temperature of the magnet coils in a persistent mode.
 18. The MRI system as recited in claim 15, further comprising a magnetic field sensor proximate the magnet coils so as to measure the present magnetic field strength of the magnetic field generated by the magnet coils, and wherein the at least one operating parameter value includes the present magnetic field strength.
 19. The MRI system as recited in claim 15, further comprising a current monitor in electrical communication with the power supply so as to measure the current generated by the power supply, and wherein the at least one operating parameter value includes the current generated by the power supply.
 20. The MRI system as recited in claim 12, wherein the magnet coils are composed of at least one of niobium, niobium-titanium, or triniobium-tin.
 21. The MRI system as recited in claim 12, wherein the mechanical cryocooler comprises one of a Gifford-McMahon (GM) cryocooler or a pulse tube cryocooler. 